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The Dosimetric Capabilities of Intensity Modulated Proton Therapy in Hippocampal Avoidance Whole Brain Radiotherapy

A Uejo

A Uejo*, B Allred , J Stoker , W Liu , J Shen , M Bues , T Daniels , S Keole , T Vern-Gross , M Buzan , Mayo Clinic Arizona, Phoenix, AZ


SU-I-GPD-T-157 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Patients treated with whole brain radiation therapy (WBRT) tend to exhibit an early decline in neurocognitive function leading to a decrease in quality of life. RTOG 0933 launched a hippocampal avoidance protocol (HA-WBRT) in an effort to reduce radiation-induced hippocampal injury, and results have shown a significant improvement in neurocognitive function preservation. This study investigates the capabilities of intensity modulated proton therapy (IMPT) for HA-WBRT and compares the IMPT results with that of photons.

Methods: VMAT and IMPT plans were created for 20 patients with previous brain treatments. The VMAT plans consisted of two full arcs and the IMPT plans consisted of a posterior-anterior field and a craniocaudal field angled 25 degrees anteriorly from axis utilizing multifield optimization. For both, a 30 Gy dose, separated into 10 fractions, was optimized to treat the whole brain while sparing the hippocampi. Treatment plans were evaluated on homogeneity index (HI), where HI=(D5%)/(D95%), inhomogeneity coefficient (IC), where IC=(D5%-D95%)/(mean dose), and V95% of the whole brain target volume.

Results: IMPT achieved better target coverage and hippocampal sparing and surpassed VMAT in all evaluated metrics. IMPT achieved a HI of 1.05±0.004 vs. 1.41±0.06 for VMAT and an IC of 0.0459±0.004 vs. 0.311±0.03. The V95% for IMPT was 97.0±0.7% and that of VMAT was 90.4±1%. IMPT saw a reduction in the mean hippocampal dose compared to VMAT from 10.2±0.3 Gy to 4.08±0.3 Gy with p << 0.001.

Conclusion: Compared to VMAT, IMPT significantly reduced the hippocampal dose by 50% or more while at the same time improved whole brain coverage and homogeneity. These improvements could lead to greater preservation of neurocognitive function and patient quality of life as well as enhanced local control. Prospective studies are warranted in order to further investigate the dosimetric value of IMPT in HA-WBRT and its impact on neurocognitive outcomes.

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